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Please note this is a comparison between Version 1 by Yoshimitsu Kiiriyama and Version 2 by Jessie Wu.
Bile acids (BAs) are produced from cholesterol in the liver and are termed primary BAs. Primary BAs are conjugated with glycine and taurine in the liver and then released into the intestine via the gallbladder. After the deconjugation of glycine or taurine by the gut microbiome, primary BAs are converted into secondary BAs by the gut microbiome through modifications such as dehydroxylation, oxidation, and epimerization.
- bile acids
- gut microbiota
- FXR
- TGR5
- SHP
- FGF
- Hippo
- Mst1
- Mst2
- MRGPRX4
- isoalloLCA
1. The Role of Modified Bile Acids in the Metabolic System
The main BA receptors are FXR and TGR5. BA binding to FXR and TGR5 regulates BA, glucose, and lipid levels. CDCA and DCA have a high affinity for FXR [1][2][3][4][64,65,66,67] and conjugated DCA acts as a ligand for FXR with NTCP—a BA transporter expressed in the cell [1][64].
FXR regulates the expression of various genes by binding to specific DNA sequences in the genome, either as a monomer or as a heterodimer with the retinoid X receptor (RXR). These specific DNA sequences are an inverted repeat of AGGTCA separated by one nucleotide (IR1), IR0, an everted repeat of AGGTCA separated by two or eight nucleotides (ER2 or 8), and a direct repeat of AGGTCA separated by one, four, or five nucleotides (DR1, 4, or 5) [5][6][7][8][9][68,69,70,71,72]. FXR also regulates epigenetic modifications. The activation of FXR recruits steroid receptor coactivator 1 (SRC1), which has histone acetyltransferase (HAT) activity in its C-terminal region, leading to the acetylation of histone H3 at the promoter of the glucagon-like peptide 1 (GLP-1) receptor [10][11][12][73,74,75]. In addition, FXR induces the expression of microRNA-29a, which downregulates DNA methyltransferases [13][76] and histone deacetylase (HDAC) 4 [14][77]. It has been shown that the percentage of common DNA binding sites of FXR between the liver and intestine is only 11% [6][69]. This suggests that the pattern of gene expression regulated by FXR is tissue-specific. Furthermore, FXR induces the expression of TGR5 [15][78].
TGR5 is a member of the G-protein-coupled receptor (GPCR) superfamily and couples to Gαs. Activated TGR5 leads to the activation of adenylyl cyclase, an increase in cyclic AMP (cAMP) concentration, and the activation of protein kinase A (PKA) and exchange proteins directly activated by cAMP (EPAC) [16][79]. LCA, DCA, CDCA, CA, and conjugated forms of these BAs activate TGR5. LCA and DCA, which are secondary BAs, and their conjugated forms were reported as the most effective TGR5 activators [17][18][19][80,81,82]. Additionally, UDCA is a weak activator of TGR5 [18][20][81,83].
1.1. BA Metabolism
FXR inhibits BA synthesis by inducing small heterodimer partner (SHP), a nuclear receptor that lacks a ligand-binding domain [21][84]. SHP interacts with and represses nuclear receptors and transcription factors, including PXR, CAR, LXR, GR, and FXR [22][23][85,86]. SHP also inhibits the activation of liver receptor homolog-1 (LRH-1), which induces CYP7A1, the key enzyme responsible for BA synthesis from cholesterol [24][87] (Figure 1A). Furthermore, FXR also inhibits the expression of CYP7A1 by inducing fibroblast growth factor (FGF) 15/19 (mouse FGF15 and human FGF19) and β-Klotho (βKL). βKL forms binary complexes with FGF receptor 4 (FGFR4) to function as a coreceptor of FGFR4 for FGF15/19 [25][88]. Transcription factor EB (TFEB) is activated by dephosphorylation and induces CYP7A1. After activating FGFR4/βKL by FGF15/19, the mechanistic target of rapamycin phosphorylates and inactivates TFEB, leading to CYP7A1 repression [26][89]. FGF15/19 also stabilizes SHP, which represses CYP7A1 and CYP8B1. CYP8B1 is a key enzyme for the synthesis of CA. The binding of FGF15 to FGFR4 activates the Hippo signaling pathway. In the Hippo signaling pathway, SHP is phosphorylated and stabilized by mammalian sterile 20-like kinase 1 (Mst1) and Mst2, homologs of the Hippo kinase in Drosophila [27][90] (Figure 1B). FXR also regulates BA transporters. FXR represses NTCP in the liver, leading to a reduction of the hepatic uptake of Bas [28][91]. On the other hand, FXR enhances BA efflux into the liver by inducing BSEP and OSTα/β [29][30][92,93]. Overall, FXR reduces excess Bas in the liver. Although TGR5 is weakly expressed in the liver, TGR5-knockout mice exhibit a decrease in total BA pool size [31][94]. Additionally, TGR5-knockout mice exhibit reduced levels of CYP7B1 and increased levels of secondary and hydrophobic Bas [32][33][34][95,96,97]. PXR is activated by LCA [35][36][98,99] and PXR regulates the expression of CYP3A and CYP2B, which convert LCA to hyocholic acid (HCA) and UDCA [37][38][39][100,101,102]. In addition, PXR indirectly suppresses the expression of CYP7A1 by inhibiting the activation of hepatocyte nuclear factor 4α, which upregulates the expression of CYP7A1 [40][41][103,104]. VDR is also activated by LCA and its metabolite, 3-keto LCA [42][43][105,106], and LCA induces the expression of CYP3A [43][44][45][106,107,108]. LXRα is weakly activated by hyodeoxycholic acid (HDCA) [46][109]. The BA pool size and its excretion are decreased in LXRα-knockout mice. LXRα-knockout mice also exhibit reduced CYP7A expression [47][110], but LXRβ-knockout mice do not [48][111].
Figure 1. Bile acid metabolism by FXR (A) Farnesoid X receptor (FXR) induces small heterodimer partner (SHP) and SHP inhibits liver receptor homolog-1 (LRH-1), which induces cytochrome P450 (CYP) 7A1 in the hepatocyte. (B) FXR induces fibroblast growth factor (FGF) 15/19 (mouse FGF15 and human FGF19) and FGF15/19 in the enterocytes. In the hepatocyte, FGF15/19 activates FGF receptor 4 (FGFR4) and its coreceptor β-Klotho (βKL), leading to the phosphorylation of transcription factor EB (TFEB) by the mechanistic target of rapamycin (mTOR). TFEB induces CYP7A1 and is inactivated by the phosphorylation. The binding of FGF15 to FGFR4 activates mammalian sterile 20-like kinases 1 (Mst1) and Mst2, homologs of the Hippo kinase in Drosophila. MST1 and 2 phosphorylate and stabilize SHP, which represses CYP7A1 and CYP8B1.
1.2. Glucose Metabolism
Glucose and insulin upregulate and downregulate FXR expression, respectively, and FXR is associated with glucose metabolism [49][50][112,113]. The association between FXR and glucose metabolism is differently affected by the fed and fasting states. In the fed state, treatment of wild-type mice with the FXR ligand decreases the expression of phosphoenolpyruvate carboxykinase (PEPCK) and glucose 6-phosphatase (G6Pase). PEPCK and G6Pase are essential for the upregulation of gluconeogenesis. In contrast, treatment of wild-type mice in the fasting state with FXR ligand increases the expression of PEPCK and G6Pase. FXR-knockout mice exhibit decreased blood glucose levels, and PEPCK and G6Pase expression are downregulated. The effects of FXR with the FXR ligand are mediated by GR [51][114]. The treatment of wild-type mice with CA reduces blood glucose levels and PEPCK and GP6ase expression. Furthermore, the treatment of SHP-knockout mice with CA increases blood glucose levels. Thus, the blood glucose-lowering effect of FXR is, at least in part, mediated by SHP [52][115]. FXR also inhibits PEPCK and G6Pase expression by inducing FGF15/19. FGF15/19 inactivates the cAMP regulatory element-binding protein (CREB), the transcription factor that induces peroxisome proliferator-activated receptor (PPAR)-γ coactivator-1α (PGC-1α) [53][116]. PGC-1α is a coactivator that induces the expression of genes involved in metabolic pathways and mitochondria by interacting with a variety of nuclear receptors [54][55][117,118]. FXR interacts with the carbohydrate responsive element binding protein (ChREBP) and suppresses the activity of ChREBP. ChREBP is a transcription factor that regulates the expression of genes controlling glycolysis and lipogenesis. ChREBP upregulates the expression of L-pyruvate kinase (L-PK), a key enzyme for glycolysis [56][119]. Activated FXR releases ChREBP and its coactivators (p300 and CREB-binding protein) from the L-PK promoter region and inhibits L-PK expression [57][120]. FXR and CDCA induce glucose transporter 4 expression in differentiated 3T3-L1 adipocytes and the hepatic cell line HepG2 [58][121]. FXR increases glucose-stimulated insulin secretion and expression by inducing the KLF11 transcription factor [50][113]. Although CDCA increases the glucose-stimulated secretion of insulin from wild-type mice, FXR-knockout mice are unaffected [59][122].
TGR5 activation increases the intestinal level of GLP-1 [60][61][62][63][64][123,124,125,126,127]. LCA, taurodeoxycholic acid (TDCA), and TLCA increases GLP-1 secretion via TGR5 activation [60][62][123,125]. GLP-1 release is mediated by TGR5 located in the basolateral membrane of L-cells [62][125]. Most active GLP-1 is degraded in the intestine and liver and 10–15% of secreted GLP-1 from the intestine reaches the pancreas via the systemic circulation [65][128]. GLP-1 induces the release and synthesis of insulin in β-cells in the pancreatic islets. The binding of GLP-1 to GLP-1 receptor activates adenylate cyclase, leading to increased cAMP levels. PKA and EPAC are then activated, which induces an increase in intracellular Ca2+ from the ER and extracellular sources via voltage-dependent Ca2+ channels [65][66][128,129]. Additionally, the activation of PKA by GLP-1 leads to the activation of pancreatic duodenal homeobox-1 protein (PDX-1), which binds to the promoter of the gene encoding insulin and induces the synthesis of insulin [65][67][128,130] (Figure 2). In contrast to TGR5, FXR suppresses GLP-1 production in L-cells by inhibiting the activity of ChREBP, which induces the expression and secretion of GLP-1 in L-cells [57][68][120,131]. Furthermore, the level of serum HCA is lower in diabetics and people with a high body mass index [69][70][71][132,133,134]. HCA induces GLP-1 by activating TGR5 and inhibiting FXR, and TGR5 activation increases serum insulin and decreases blood glucose levels [69][132].
Figure 2. The regulation of insulin secretion and synthesis via TGR5 and FXR. TGR5 located in the basolateral membrane of L-cells mediates glucagon-like peptide 1 (GLP-1) secretion from L-cells, and secreted GLP-1 from the intestine reaches pancreatic β-cells via the systemic circulation. The binding of GLP-1 to GLP-1 receptor (GLP1R) increases cyclic AMP (cAMP) levels, leading to the activation of protein kinase A (PKA) and exchange proteins directly activated by cAMP (EPAC), which induces an increase in intracellular Ca2+ and insulin secretion. In addition, the activation of PKA by GLP-1 leads to the activation of pancreatic duodenal homeobox-1 protein (PDX-1), which binds to the promoter of the gene encoding insulin and induces the synthesis of insulin. FXR suppresses GLP-1 production in L-cells by inhibiting the activity of a carbohydrate responsive element binding protein (ChREBP) that induces the expression and secretion of GLP-1 in L-cells.
1.3. Lipid Metabolism
Dietary lipids (triglycerides [TGs], cholesterol, and phospholipids) are absorbed and incorporated into chylomicrons in the intestine. Chylomicrons are then converted to very low-density lipoprotein (VLDL) in the liver. VLDL transports lipids from the liver to the systemic circulation. VLDL is then converted to HDL and returned to the liver [72][135]. In the process of lipogenesis, acetyl-CoA carboxylase (ACC) converts acetyl-CoA to malonyl-CoA, and fatty acid synthase (FAS) converts malonyl-CoA to palmitate. Stearoyl-CoA desaturase (SCD) mediates the conversion of saturated fatty acids (FAs) to monounsaturated FAs, and the newly-produced FAs are then stored as TGs. Sterol regulatory element-binding protein 1C (SREBP1C) is a transcription factor that induces lipogenic genes, including FAS, ACC, and SCD [73][74][136,137]. FXR-knockout mice exhibit an increase in SREBP1C, SCD-1, and FAS and also exhibit increased levels of serum and liver TGs, serum cholesterols, and serum free FAs (FFAs) [52][115]. Additionally, CA feeding decreases the serum levels of TGs, cholesterol, and FFAs in mice [52][75][115,138]. CA and CDCA decrease the expression of FAS, ACC, SCD, and SREBP1C [75][138]. SHP induced by FXR suppresses lipogenesis by suppressing lipogenic genes via the inhibition of LXR, which induces SREBP1C expression [75][76][138,139]. Furthermore, FXR induces the expression of carboxylesterase 1 (CES1), which reduces hepatic TGs by inducing TG hydrolysis to release FFAs. Released FFAs activate PPARα, leading to the enhancement of FA oxidation [77][78][140,141].
2. Role of Modified BAs in the Immune System
The liver in FXR-knockout mice exhibits an elevated expression of pro-inflammatory genes, such as interleukin-1 (IL-1), IL-6, interferon-γ (IFN-γ), inducible nitric oxide synthase, and cyclooxygenase-2. FXR activation inhibits the expression of these genes by inhibiting the activity of nuclear factor-κB (NF-κB), and also has an anti-inflammatory function [79][142]. Moreover, CDCA inhibits the production of IL-1, IL-6, and tumor necrosis factor α (TNFα) by human monocytes [80][143]. FXR activation by agonists leads to conjugation with small ubiquitin-like modifier (SUMO) 2. SUMOylated FXR interacts with NF-κB and nuclear receptor corepressor (NCOR) 1 or NCOR2, also called silencing mediator of retinoic acid and thyroid hormone receptor. NCORs are transcriptional corepressors that repress pro-inflammatory gene expression in the liver [81][82][83][84][144,145,146,147]. FXR also upregulates SHP. SHP overexpression inhibits NF-κB activation and leads to the downregulation of pro-inflammatory genes in the liver. In contrast, SHP deletion activates NF-κB. Thus, FXR can suppress NF-κB activation by inducing SHP [85][148]. The inhibitory effect of SHP on NF-κB is mediated by the binding of SHP to NF-κB [86][149].
Macrophages express both FXR and TGR5. Macrophages from FXR-knockout mice exhibit enhanced release of pro-inflammatory cytokines [84][147]. CDCA and FXR agonists reduce IL-6 expression in macrophages. However, IL-6 expression is unaffected by FXR agonists in FXR-knockout mice [87][150]. Activation of TGR5 leads to the suppression of pro-inflammatory cytokines from LSP-stimulated macrophages [88][151]. The activation of macrophage TGR5 by INT-777, a TGR5 agonist, inhibits pro-inflammatory cytokine production. This inhibition is mediated by cAMP signaling, followed by NF-κB inhibition [88][151]. Furthermore, TGR5 activation promotes macrophage polarization from M1 (pro-inflammatory phenotype) to M2 (anti-inflammatory phenotype) macrophages and reduces pro-inflammatory cytokines [89][90][91][152,153,154]. In contrast, macrophage differentiation is unaffected by FXR-knockout [87][150].
Dendritic cells are antigen-presenting cells that play an essential role in the adaptive immune system and express TGR5. DCA suppresses lipopolysaccharide (LPS)-induced expression of pro-inflammatory genes, including IL-1, IL-6, and TNFα in dendritic cells. Furthermore, TGR5-knockout mice exhibit a recovery of LPS-induced expression of pro-inflammatory genes. These inhibitory effects of TGR5 are mediated by the repression of NF-κB via TGR5–cAMP–PKA signaling [92][155].
Recent studies have shown that 3-oxoLCA and isoalloLCA regulate T cell differentiation. IsoalloLCA triggers oxidative phosphorylation in mitochondria, leading to the production of reactive oxygen species (ROS). ROS from the mitochondria, not from the cytoplasm, induces forkhead box P3 (FOXP3). FOXP3 is a transcription factor and a master regulator of the development of regulatory T cells (Tregs), which release immunosuppressive factors. This 3-oxoLCA acts as an inverse agonist of retinoid-related orphan receptor γt (RORγt), leading to the inhibition of the development of T helper 17 (Th17) cells, which release pro-inflammatory factors. Both 3-oxoLCA and isoalloLCA are not produced in germ-free mice [93][94][156,157]. It has been reported that Odoribacteraceae strains can produce isoalloLCA via 5α-reductase and 3β-HSDH. Moreover, isoalloLCA shows selective antibacterial activity against gram-positive multidrug-resistant pathogens [95][56].